A listen before talk (LBT) technology is known in the related art. In the LBT technology, when a plurality of communication carriers or wireless communication systems share a frequency band, a wireless communication device senses the usage status of the frequency band before performing transmission and determines whether or not it is possible to perform transmission on the basis of the carrier-sense result in order not to cause radio interference with other wireless communication devices that share the frequency band. In an LBT technology for a wireless LAN called Wi-Fi (registered trademark) as one of the LBT technologies, carrier-sense is performed during a fixed waiting time plus a randomly determined waiting time to stochastically avoid collisions (for example, see Non-Patent Literature 1). The randomly determined waiting time is called a backoff time (Backoff).
FIG. 15 is an explanatory diagram of a related-art Wi-Fi LBT technology. As shown in FIG. 15, before performing transmission, a wireless LAN device performs carrier-sense during a time which is the sum of a fixed waiting time (a distributed inter-frame space (DIFS)) and a randomly determined waiting time (a backoff time). The backoff time is determined according to a backoff value randomly selected from a range of 0 to a contention window (CW) with a uniform probability. When the wireless LAN device has not received an acknowledgement (ACK) signal within a certain time for data that the wireless LAN device has transmitted at transmission #N (i.e., when a reception failure has occurred), the CW is enlarged to increase a probability that the backoff time becomes greater (where Backoff=Random([0, CWmin*2(n−1)−1]); n=1, 2, . . . ) in order to lower a probability that packets collide at transmission #N+1. The CW is increased twofold each time. An initial value of the CW is 15 and a maximum value of the CW is 1023. When an ACK signal has been received for data of the transmission #N+1, the enlarged CW returns to the initial value at transmission #N+2 (i.e., Backoff=Random([0, CWmin−1]); n=0). In the case of Wi-Fi, whether reception of the previous transmission #N has been successful or has failed must be determined before starting LBT for the transmission #N+1. The wireless LAN device transmits data to a single destination, performs management on whether or not an ACK signal has been received from the single destination, and determines a backoff value for a next transmission according to whether or not an ACK signal has been received from the single destination. The maximum time length of data transmission is set as a maximum transmit duration (MTD). In Japan, the maximum time length is 4 ms.
Licensed-assisted access (LAA) is being studied in the standardization of specifications for a next generation mobile communication system in the third generation partnership project (3GPP) (for example, see Non-Patent Literature 2). LAA is a technology for performing long term evolution (LTE) communication while sharing a frequency band with another wireless communication system. A wireless station (a base station (eNB) or a terminal (UE)), to which LAA is applied, has an LBT function to avoid radio interference with another wireless communication system such as a Wi-Fi communication system which uses the same frequency band.
The LBT function of LAA performs the following processes.                Comparing the power level of a frequency band in use, which has been acquired by carrier-sense, with a threshold value and determining that the status is busy if the power level of the frequency band in use is greater than the threshold value and the status is idle if the power level is equal to or less than the threshold value.        Performing transmission if the status is kept idle for a set waiting time which is the sum of a fixed waiting time and a randomly determined waiting time (a backoff time (Backoff)).        